Compression Testing

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enut

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I am looking for a sanity check, I checked the compression on my 98 Challenger 1800.
I measured exactly 100psi in all 4 cylinders. The engines ran fine all summer, but I wanted to do a compression check since I purchased the boat at the start of this boating season and I wanted to get an overall condition of the engines before storing the boat for the winter.

As the attached pictures show, there is a 26 inch hose attached to the gauge, plus another 8 inch hose that attaches by quick connect and has the correct fitting for the spark plug hole.

The battery is a good battery and has a good charge, and yes I did put the throttle all the way up.
The engines do start up with no problems.

This is from Harbor Freight item # 95187. Palerider's gauge looked similar in his post but he may have a slightly different kit. I did connect the quick connect fitting to a compressor and the gauge did match the reading of the gauge on the compressor, but this was without the last 8 inch adapter that is used to go into the spark plug hole.

Could my engines have got that low a compression, but all 4 are identical? The boat did run fine all summer.





Compression_Tester.jpgCompression_Tester_Gauge.jpg
 
Yes your compression can be that low and it still run fine. Fine for your experience with a boat with bad engines. But not fine for how the boat is supposed to perform. I just saw a ski this summer run on 90 psi and still ran fine, on the trailer. If I would have taken it to the water the low compression would have shown up immediately. If your compression gauge was leaking the pressure would start to fall as soon as you stop cranking. Sounds like you've got a winter project. If you need help with the rebuild or have questions along the way, don't hesitate to post them on here. If you do a top end rebuild only, make sure you replace the crank, seals, and bearings. Personally my plan when that time comes is just to go with a sbt full engine exchange.
 
I looked up that Tester and there are some pretty bad Reviews on it, saying it is highly unaccurate and worthless. I would buy a worthy tester and spend the money and try it again. Im not saying that is the problem, but I am just stating thatyou should look into it. Read the Reviews at Harbor Freight and see what you think.
 
I have a similar gauge from Harbor Freight, 66216, it doesn't have the long hose. I checked it against a recently calibrated Snap On gauge and it was dead on. However with that being said, before you get too excited you should check your compression again with a known good gauge. You can borrow one, with a deposit at Advance or Auto Zone.

http://www.harborfreight.com/compression-test-kit-66216.html

Lou
 
That's WAY too much hose. You could be loosing 20 psi worth of volume in it.

I'm going to suspect the gauge because at 100 psi... the engines wouldn't be running well. (Hard starting, poor idle in the water, and low top RPM)

If you go to your local auto parts store... a lot of times they "Rent" tools. Get a different gauge, and check it again. OR... go to Harbor freight, and get their $20. They work just fine.
 
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Def recheck with a known good gauge as these guys have suggested. Mine is not a HF gauge...lol...
I`m pretty sure mine is an Equus and I think I bought it from Summit Racing, lol, def not the best but works and is within 1-2+-psi compared to the Snap On gauge.
I only use it to see if the engine is healthy and close. I have Used Snap ON when wanting to know exact compression in V8`s etc...
 
I use a motion pro gauge. The price is moderate $140-$150 and you still get a good qaulity product since they make tools for the recreation product people. That guage kit is made to 2 and 4 stoke small engines. I know my HF one was bad since I was testing it on everthing I owned and the readings were wrong nothing should be running with this reading. The rental is best to use to vefify as long as you don't get a beatup one. I have seen bad rental tools. That is a long hose, doc is right you are getting a bad reading with that long of hose. That small hose you have in the pic is the lenth you should be using.
 
I'm going with your gauge is bad, there is no way that you'd have exactly 100 per hole. Look your paying $30 for a tester, a good CALIBRATED gauge is going to be a fortune, the good compression testers don't have a calibrated gauge I'd bet. That being said I have the same HF unit with the long hose. It's modeled after the Snap-On MT-308L. I borrowed the Strap-On from a buddy and compared it to my HF when I first got it and the readings were the same. The length of the hose should have no effect on the reading, I could see if you were going like 10 feet but your really close, less than 2 feet. I don't think you could fit a shorter one in on a 787 in a ski, a boat no problems. Go back and buy a new one and return the broken one for your money back next week. They're all good until they take a good drop!

171.jpg


http://buy1.snapon.com/products/diagnostics/mt308l.asp?partno=MT308L&dir=catalog

http://www.ebay.com/itm/MT-308L-SNA...156?pt=LH_DefaultDomain_0&hash=item43b451cf24
 
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I didnt realize a long hose could effect the gauge. I did a test on my 720 and came up with like 137 and 140. I did notice my tester gauge did drop with about a minute. Should it hold the pressure longer then that. My gauge looks like the one at top of post with the long hose and a short one that connects.
 
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